On Facebook Messenger how can I tell who's online?

I'm not using Facebook, nor the FB Messenger app. But from what I gather, you should find a list of your 'active/online' 'friends' somewhere...

HOWEVER: what I do know is this: the app received record 1-star ratings. And one of the reasons for that is that many many many people appear 'online' even when they aren't actually online. Some even who do not have the app installed (anymore) are listed as online.

Additional fun is caused by the fact that people can be online but hide their status...

In other words, realistically speaking the answer to your question is: you can't!

I'm not using Facebook, nor the FB Messenger app. But from what I gather, you should find a list of your 'active/online' 'friends' somewhere...

HOWEVER: what I do know is this: the app received record 1-star ratings. And one of the reasons for that is that many many many people appear 'online' even when they aren't actually online. Some even who do not have the app installed (anymore) are listed as online.

Additional fun is caused by the fact that people can be online but hide their status...

In other words, realistically speaking the answer to your question is: you can't!